The present invention relates to disc drive storage systems and, more particularly, to a disc drive storage system having a slider with angled leading rails and a notched cavity dam.
Disc drives of the "Winchester" type are well known in the industry and include hydrodynamic (e.g. air) bearing sliders which carry recording heads for communicating with the disc surface. Each slider has an air bearing surface which faces its respective disc surface. The bearing clearance between the slider and the disc surface at the recording head is an important parameter to disc drive performance. It is desired to minimize variation in the head clearance or "flying height".
One source of flying height variation in a disc drive is a variation in the disc surface velocity from the disc inner diameter (ID) to the disc outer diameter (OD). Another source is from normal process variations of manufacturing parameters including those related to external loading of the slider, such as the suspension preload and the suspension "pitch static angle" (PSA), and the length of a leading taper which is formed at the leading edge of the bearing surface of the slider.
Self-loading or "negative pressure" air bearing (NPAB) surfaces have been used in the disc drive industry for many years for reducing flying height variation. Self-loading air bearing surfaces have several common features, including the leading taper, a pair of raised side rails, a cross rail and a sub-ambient pressure cavity. The leading taper is lapped onto the end of the slider that is opposite to the recording head. As the disc rotates, the disc drags air under the slider along the air bearing surfaces in a direction approximately parallel to the tangential velocity of the disc. The leading taper pressurizes the air as the air is dragged under the slider by the disc surface. Disc pressurization creates a lifting force that causes the slider to lift and fly above the disc surface. The lifting force increases the bearing load capacity of the slider. An additional effect of the leading taper is that the pressure distribution under the slider has a peak near the taper end or "leading edge" due to the high compression angle of the taper, and a second peak near the recording end or "trailing edge" due to a low bearing clearance required for efficient magnetic recording. This dual-peak pressure distribution results in an air bearing with a high pitch stiffness. A high pitch stiffness results in a head flying height which is relatively insensitive to process variations in the suspension pitch static angle ("PSA").
The pair of raised side rails extends downstream from the taper. The side rail closest to the disc hub is called the "inner rail" and the side rail closest to the disc rim is called the "outer rail". The rails may extend from the taper to the trailing edge, in which case there are usually two heads, with one head mounted near the trailing end of each rail. Alternatively, there may be a single head mounted on a center rail or island positioned at the trailing edge. In this case, the raised side rails are typically truncated prior to the trailing edge. With the dual-peak pressure distribution, if the side rails are positioned along the side edges of the slider, the slider develops an air bearing with a high rolling stiffness and an insensitivity to the suspension roll static angle ("RSA").
The sub-ambient pressure cavity is positioned between the inner and outer rails. The sub-ambient pressure cavity is open to the atmosphere at the trailing edge, and is bounded by the cross rail located near the leading edge. The cross rail extends between the inner rail and the outer rail. The cross rail provides an expansion path for the air to depressurize as it is dragged into the sub-ambient pressure cavity by the disc velocity. The expanded air in the cavity provides a self-loading force which forces the slider toward the disc surface. The counteraction between the increased bearing load, the preload force and the self-loading force provides the air bearing with a high vertical stiffness and an insensitivity to variations in the suspension preload.
A further source of flying height variation is from environmental conditions such as altitude. Air bearing fly height loss at high altitude is becoming an increasing concern, especially in mobile applications with magnetoresistive heads and low nominal head-disc separation. In these applications, the operating environment for any single disc drive can include varying atmospheric pressure such as when an end user at sea level carries a notebook computer onto an airplane that is pressurized to 10,000 feet in flight. The air bearing performance over varying atmospheric pressure is governed by the Reynolds equation. The air bearing load can be obtained by solving the Reynolds equation for pressure and then integrating the pressure over the bearing area. From the Reynolds equation, the bearing load can be shown to be a function of the bearing number, which is a measure of the compressibility of the air under the air bearing surface. The bearing number is proportional to the disc surface velocity and is inversely proportional to the atmospheric pressure and the square of the characteristic film thickness. For calculating the lift force, the characteristic film thickness is the close point flying height of the slider. For calculating the self-loading force, the characteristic film thickness is the cavity depth.
The bearing load is determined by the combination of the load carrying effects of the ambient pressure and the bearing induced compressibility which is determined by the disc surface velocity and the characteristic film thickness. When the bearing number is small, either the bearing's ability to compress or expand air is not saturated or the ambient pressure effect is saturated. Therefore, the bearing load is dominated by the compressibility effect and is independent of ambient pressure. When the bearing number is large, the bearing's ability to compress or expand air is saturated and the bearing load is proportional to ambient pressure. The bearing number for the lifting force is very large for all disc drive applications.
The ambient pressure is reduced with increasing altitude. Therefore, the loss of lifting force is substantial at high altitude, where ambient pressure is low. Loss of lifting force is equivalent to loss of fly height. Also, the mean free path of air increases with altitude. This increased mean free path increases side leakage of air along the side edges of the rails and further lowers air bearing fly height.
Fly height sensitivity to altitude can be reduced by compensating lifting force loss with self-loading force loss. If the self-loading force loss is the same as the lifting force loss, then the fly height is virtually insensitive to altitude change, without considering the pitch angle and roll components of the slider. The cavity bearing number is small if the cavity depth is deep or if the disc surface velocity is low. For a low cavity bearing number, the self-loading force is not reduced with increasing altitude or with decreasing ambient pressure. Since the lifting force is reduced with increasing altitude and the self-loading force is not reduced with increasing altitude, the fly height loss with high altitude is severe.
When the cavity depth is shallow or when the disc surface velocity is high, the cavity bearing number is large. Thus, the self-loading force is greatly reduced at high altitude, which can compensate for the lifting force loss and thereby minimize fly height loss at high altitude. For low disc surface velocity applications, such as with mobile products, the typical range of cavity depth for minimizing fly height loss with altitude is about 2-4 micrometers.
In addition to accommodating exposure to a wide range of ambient pressures, mobile disc drive products have inherent constraints related to minimizing power consumption and improving reliability. One measure of reliability is the minimum number of successful contact start and stop ("CSS") cycles in a particular disc drive. Power consumption and reliability are not necessarily independent of one another. Power saving strategies can include shutting down the disc spindle motor during long idle times, thereby causing an additional CSS cycle without fully shutting off the disc drive.
One way to minimize power consumption is to spin the disc at the slowest speed allowable to meet the data rate requirements for the disc drive. Smaller form factors of mobile disc drive products and reduced spindle speeds combine to create large cavity bearing numbers. As described above, it is then necessary to have a shallow cavity depth for minimizing fly height loss with altitude. One method for improving both power consumption and CSS performance is to use a low preload force.
When the slider is a rest on the disc surface, asperities on the slider and the disc surface are in contact with one another. Two types of forces are created by this asperity contact, "stiction" forces and meniscus shear forces. Stiction forces are friction forces between the slider and the disc, which oppose relative motion between the slider and the disc as the disc starts to rotate. The contact element of stiction is determined by the microscopic contact area, the local material stress and the molecular interface bonding. This classic static friction component is substantially proportional to the applied normal force on the slider and can be reduced with lower suspension preload.
The combined surface roughness of the slider and the disc forms a capillary channel in which water condensate and excess disc lubrication act as a squeeze film. The meniscus shear force arises from the viscosity of the squeeze film and opposes relative motion between the slider and the disc. The squeeze film effect dominates when the capillary channel is filled. When the slider-disc interface is not saturated with a squeeze film as the motor starts, the maximum energy required by the motor can be reduced by reducing the preload and thus the stiction between the slider and disc.
As the disc continues to rotate and the squeeze film is de-wetted from the interface, work, which is proportional to the preload, is created from the dynamic friction of the sliding contact between the surfaces until the air bearing lift force is sufficient to separate the slider and the disc. The net work is the energy that is not dissipated by heat transfer. The net work that exceeds the fracture strain energy of the interface creates wear. Minimizing wear during a contact start-stop cycle is a key objective for satisfactory CSS performance. Because the work is proportional to the preload, wear can be reduced with lower preload. A typical maximum preload for adequate CSS performance is 5 gmf.
Wear can be further reduced by using a shallow cavity depth. The shallow cavity depth, such as desired for fly height insensitivity to altitude, generates a higher self-loading force than that for a deeper cavity. The higher self-loading force requires more air bearing rail surface area to maintain the same flying height when the disc is rotating at operating speed. At low disc speeds during spindle start, the cavity bearing number is quite low and there is little self-loading force generated, which translates to a lower take off speed for the slider, less distance in contact, and less wear energy.
For mobile disc drive applications, it is therefore desirable to provide the air bearing surface with a shallow cavity depth and a low preload. For the current range of disc surface velocities, a shallow leading taper angle of approximately 1.0 milliradians provides a head flying height that is essentially constant across the disc from ID to OD. However, because of process limitations, the leading taper angle is typically greater than 4.0 milliradians. These higher taper angles increase the higher surface pressurization from ID to OD. This tends to translate to an increased head flying height at the disc OD, thereby reducing the magnetic recording efficiency and the storage capacity of the disc drive.
A common technique for counteracting the pressurization characteristics of a high taper angle is to reduce the surface area of the leading taper, which also reduces the flying pitch angle of the slider. The reduced pitch angle better retains the air under the leading taper, forms the desired dual-peak pressure distribution and results in a high air bearing pitching stiffness that maintains a head flying height that is relatively insensitive to normal variations in suspension PSA.
Slider air bearing surface configurations have been developed that include a notch which reduces the surface area in a central region of the leading taper for reducing sensitivity to altitude and improved CSS performance in some applications. However, it has been found by the present inventors that these configurations can have low interface reliability when used in a mobile product with a shallow cavity depth and a low preload. The present invention provides a solution to these and other problems, and offers other advantages over the prior art.